1. Technical Field
The present invention relates to a preamble sequence detection and integral carrier frequency offset estimation method for OFDM/OFDMA wireless communication system, particularly to a preamble sequence detection and integral carrier frequency offset estimation method for OFDM/OFDMA wireless communication system using frequency-domain filtering to calculate the required correlation values which can drastically reduce the high computational complexity of the original theoretically optimal solution.
2. Description of Related Art
The main problems that the inventors of the present invention attempts to remedy are about the traditional approaches for estimation of integral carrier frequency offset (CFO) (i.e. the integral part of the ratio of the CFO to subcarrier spacing) and detection of base station (BS) identity. The traditional approaches usually address only one of these two topics or handle them separately, resulting in inadequate efficiency and innate defects. Generally, the prior approaches for CFO estimation and/or BS identity detection can be classified into the following three types:
1. Calculate certain correlation coefficients between the received signal and some particular signals in the time domain. Based on the calculated correlation coefficients, estimate the CFO or detect the BS identity according to the related coefficients. The particular signals used in calculating the correlation coefficients are set according to the range of the CFO to be estimated and the set of codes for the BS identity to be detected. Among this type of approaches are the prior methods disclosed in [US 2006/0133321 A1, “Method and apparatus for cell search in wireless communication system,” Jun. 22, 2006], [US 2006/0126491 A1, “Cell search apparatus and method in a mobile communication system using multiple access scheme,” Jun. 15, 2006], [US 2006/0114812 A1, “Method and apparatus for embodying signal in mobile communication system and method for searching cell using the same,” Jun. 1, 2006] and in the technical literature, [T. M. Schmidl and D. C. Cox, “Robust frequency and timing synchronization for OFDM,” IEEE Trans. Commun., vol. 45, pp. 1613-1621, December 1997].
2. Convert the calculation or part of the calculation in time domain as described above into equivalent calculation in the frequency domain. This approach is taken in the prior method disclosed in [US 2005/0271026 A1, “Method and apparatus for detecting a cell in an orthogonal frequency division multiple access system,” Dec. 8, 2005].
3. Calculate some kind of differential correlation in the frequency domain. Then estimate the CFO or detect the BS identity based on the correlation coefficient. Among this type of approaches are the prior methods disclosed in [US 2005/0157637 A1, “Cell search method for orthogonal frequency division multiplexing based cellular communication system,” Jul. 21, 2005], [US 2006/0078040 A1, “Apparatus and method for cell acquisition and downlink synchronization acquisition in a wireless communication system,” Apr. 13, 2006.], [US 2006/0133321 A1, “Method and apparatus for cell search in wireless communication system,” Jun. 22, 2006] and in the technical literature, [Y. H. Kim, I. Song, S. Yoon, and S. R. Park, “An Efficient Frequency Offset Estimator for OFDM Systems and Its Performance Characteristics,” IEEE Trans. Vehicullar Tech. vol. 50, no. 5, pp. 1307-1312, September 2001], [M.-H. Hsieh and C.-H. Wei, “A Low-Complexity Frame Synchronization and Frequency Offset Compensation Scheme for OFDM Systems over Fading Channels,” IEEE Trans. Vehicular Tech. vol. 48, no. 5, pp. 1596-1609, September 1999], and [H. Lim and D. S. Kwon, “Initial Synchronization for WiBro,” in Asic-Pacific Conf Commun., 2005, pp. 284-288].
In addition to the foregoing inadequate efficiency and innate defects, the prior methods have the following additional shortcomings:                1. These mentioned methods are mostly heuristic designs, without well-rounded theoretical foundation. Thus the selection of their parameters are typically conducted through heuristic or trial-and-error approaches, without firm guarantee of performance.        2. Concerning computational complexity, the first two types of approaches usually require a high complexity and the third type usually lower. However, a method that can perform well at an intermediate or even lower complexity than the third type of approaches is in want.        3. The design of some methods does not conform to the IEEE 802.16e OFDMA/WiMAX standards. Rather than commenting on each of such methods, we only briefly discuss several representative examples. For one example, in the technical literature, [T. M. Schmidl and D. C. Cox, “Robust frequency and timing synchronization for OFDM,” IEEE Trans. Commun., vol. 45, pp. 1613-1621, December 1997], it is assumed that there are two consecutive, known pilot symbols, with their total length approximately shorter than the channel coherence time. Moreover, only the CFO is considered but not the BS identity. Therefore, the set conditions are unsuitable. For another example, in the technical literature [M.-H. Hsieh and C.-H. Wei “A Low-Complexity Frame Synchronization and Frequency Offset Compensation Scheme for OFDM Systems over Fading Channels,” IEEE Trans. Vehicular Tech. vol. 48, no. 5, pp. 1596-1609, September 1999], it is assumed that there is some kind of relation between the channel responses at neighboring subcarriers. Again, only the CFO is considered but not the BS identity. Therefore, the set conditions thereof are also unsuitable. For yet another example, the algorithm described in [US 2005/0271026 A1, “Method and apparatus for detecting a cell in an orthogonal frequency division multiple access system,” Dec. 8, 2005] is unclear and over-abbreviated. It provides no definite design geared to the signal characteristics of IEEE 802.16e OFDMA/WiMAX. Thus this prior algorithm is impracticable in this regard.        